In prior U.S. Pat. No. 3,780,011, assigned to the assignee hereof, there is described and claimed a catalyst and method in which the catalyst is prepared by treating a mixture of a support such as silica and chromium trioxide with an ester of titanium, boron or vanadium or mixtures while fluidizing the resulting bed, followed by activating the bed with a dry gas that contains oxygen at elevated temperatures. This catalyst is useful in producing polyolefins and particularly polyethylene with controllable viscoelastic properties. When the ester is boron the activity in ethylene polymerization is satisfactory but preferably should be higher and the polyethylene has narrow molecular weight distribution. In the present invention the catalyst which contains boron as well as chromium has greatly increased activity while still producing polyethylene having a narrow molecular weight distribution.